MCF (monolithic crystal filter) based on quartz technology operating in the fundamental shear mode have been used over the past half century for radio communication applications. The center frequency of these filter are limited to about 10-250 MHz, due to the fabrication difficulty. In the last decade, the rapid progress in the wireless communication has created a strong demand for high performance Gigahertz filters with small dimension and low power consumption.
Currently, the commercially available monolithic crystal filters are usually limited to center frequency of about 250 MHz due to the fabrication difficulties. These relatively low frequency filters showed a rather high insertion loss of 6-7 dB when good out-of-band rejection of about 60 dB is required. At high frequency, the difference in the strength of the first two undesirable anharmonic modes observed in a resonator are usually less than 10 dB from the desirable fundamental shear mode. The suppression of these anharmonic modes has become a critical issue for a high performance Giga Hertz MCF.
To extend the existing MCF technology into the GHz range is not trivial. For a low frequency MCF, a good out-of-band rejection may be achieved by designing a quartz filter supporting only fundamental shear mode. However, at high frequency, greater than 1 GHz, a single mode quartz filter will have the dimension of smaller than a few microns by a few microns, and a gap of 1 micron or less. The tolerance for the fabrication error for such a filter may be costly and difficult to attain with conventional processing. Therefore, a highly effective technique for suppression of anharmonic modes is necessary.
Thus, what is needed is a high performance Giga hertz MCF capable of effectively suppressing anharmonic modes.